There are known so-called hollow particles with a structure having a void in the interior thereof. Hollow particles have been employed as a functional material through application of their voids. For instance, there are cited a case of utilization as a microcapsule by encapsulating various kinds of material within voids and a case of addition to sheet paper as light-scattering material by employing diffused reflection within the voids. There is also used as a heat insulating material by employment of the insulation effect of the voids.
Such hollow particles were often made, in their early stage, by applying a physical action employing gas, heat or the like to polymeric particles. For instance, a foaming agent or volatile material such as butane is preliminarily included in polymeric particles and allowed to foam or gasify, forming the voids. There was also a technique in which liquid is allowed to permeate into polymeric particles containing an alkaline-swellable material and the contained material is allowed to swell to form voids, as described in JP-A No. 56-32513 (hereinafter, the term JP-A refers to Japanese Patent Application Publication).
These methods of applying a physical action to polymeric particles often resulted in a complex production process or produced problems such that they took some time and effort to make uniform the shape or size of voids. Thereafter, there was discovered a method of polymerizing a polymerizable monomer to form hollow particles. Such a polymerization method produced hollow particle uniform in void form or size and became an effective means for volume production of uniform quality.
There was disclosed, for example, a method in which polymerization was performed in an aqueous medium to form resin particles and concurrently phase separation and polymerization shrinkage were performed in the interior of the formed particles to form voids, as described in JP-A No. 62-127336. There was also disclosed a technique in which polymerization was conducted in stepwise decreased addition of a monomer containing a carboxy group to form hollow particles having a triple-layered structure, as described in JP-A No. 6-248012.
There was also disclosed a method in which a cross-linkable a mixture of a monomer, a polymerization initiator and an organic solvent is dispersed in an aqueous medium and suspension polymerization is performed to form single-layered resin particles encapsulating the organic solvent, followed by removal of the organic solvent by spontaneous vaporization or evacuation to form hollow particles, as described in JP-A No. 2002-80503.
Recently, there are placed expectations of making hollow particles through polymerization methods, as set forth above.